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The End of a World - The Cretaceous/Paleogene boundary in Denmark

The Mesozoic, the era of reptiles, was characterized by high sea levels and a warm climate. It came to an abrupt end about 65.5 million years ago, after 165 million years of dominance by the dinosaurs. This marks the border between two geological eras: the Cretaceous and the Paleogene and is therefore called the ’Cretaceous–Paleogene boundary'. In the past, it was called the ' Tertiary ' instead of ' Paleogene ' and so it is also known as ‘the Cretaceous-Tertiary boundary '. On this boundary the dinosaurs died out and with them many other plants and animals. To be exact, on land, all animals heavier than 25 pounds died: the dinosaurs, the flying reptiles (pterosaurs, etc.) and the larger mammals!

In the world's oceans about 50% of all species died, including the ammonites and belemnites (which occurred world-wide in large numbers during the Mesozoic) and many marine reptiles as well as a large part of the plankton. Such an event, where a large part of life on Earth dies out, geologists call a mass extinction. With this mass extinction on the Cretaceous–Paleogene boundary, when approximately 50% of all species on Earth became extinct, a world came to an end: the world of the dinosaurs. These extinctions also meant the beginning of a new world, however. A new era: the era of the mammals, and the world as we know it.

Since the discovery of this mass extinction, geologists are fascinated by this important event in the history of life. In the history of life on Earth, such a mass extinction has occurred only 5 times;  the mass extinction on the Cretaceous–Paleogene boundary is the most recent one, and one of the most abrupt.

One of the aspects that makes this event so fascinating is the speed with which it happened. Geologists have divided the history of the Earth into many eras, but the boundary layers are not sharp and clearly visible in rocks. The Cretaceous–Paleogene boundary is an exception. In many places where the upper Cretaceous and lower Paleogene are visible, the boundary can be seen as a sharp transition, a change in rocks. In places where the border has remained well preserved you can often recognize a thin clay layer. Overnight, nearly all the lime-producing plankton died out, resulting in rocks changing from lime or marl to clay from one millimeter to the next!

This abrupt transition has kept geologists busy for decades. The idea of a very abrupt event in Earth's history did not seem to combine with the idea of gradualism (the concept that all geological developments are the result of slow, gradual processes and mechanisms), which has been the common theory amongst scientists since the end of the 18th century. Over the years there have been many theories explaining the mass extinction. At first the causes were  thought to be very extensive volcanism, or changing ocean currents or sea level increases and decreases, but none of these theories could explain the abruptness of this mass extinction on the Cretaceous–Paleogene boundary.

In the 1970s, two American scientists, the physicist Luis Alvarez and his son geologist Walter Alvarez, investigated the formation rate of rocks near the Italian town of Gubbio. The section which they studied turned out to consist of the Cretaceous–Paleogene boundary. For their research they measured the iridium concentrations in the rocks because this can be used to calculate the speed at which these rocks were formed.

However, when they measured the iridium concentration of the clay layer on the Cretaceous–Paleogene boundary, they could hardly believe their measurement results. It contained nearly 30 times the normal concentration of iridium! Father and son Alvarez soon came to the conclusion that this was not a measurement error, but that there was something very different  going on. The only way to explain such a large amount of iridium was the impact of a large meteorite. An impact that turned out to be responsible for the extinction of 50% of life on Earth.

In order to prove this theory, however, they had to be able to demonstrate that the clay layer on the Cretaceous–Paleogene boundary contained a high iridium concentration not only in Gubbio, but all over the world. To do this, they went looking for good sections, which were already completely and extensively studied. They soon ended up in Denmark. In different places in Denmark, both on the Jutland peninsula as well as on the island of Seeland,  there are beautiful outcrops of the Cretaceous–Paleogene border (Stevns Klint, Nye Klov, Kjolby Grove and the Dania quarry). One section in particular was very suitable for their purpose: Stevns Klint! This section had recently been extensively studied (see, for example, Christensen et al., 1973) and so father and son Alvarez travelled to Denmark to study the Cretaceous–Paleogene boundary there.

Here, too, they took samples of  the clay layer on the border, which in Denmark is called the Fiskeler, or ' fish-clay ' because there are relatively many fish remains (scales and bones) found in it. Exactly as Luis and Walter expected, this clay layer, too, contained a very high concentration of iridium. In fact, the iridium concentrations were even much higher than in Italy! Not much later, on June 6th, 1980, they published their results in the leading Science journal with the now-famous article "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction", in which they determined that the impact of a large asteroid was the most likely cause of the mass extinction on the Cretaceous–Paleogene boundary.

The impact of an asteroid, the size of which has been estimated to be about 10-12 km, would have resulted in a series of global disasters: earthquakes, tidal waves,  intense heat and forest fires. The impact would have caused a vast dust cloud to be spread over the Earth, which blocked the sunlight and produced acid rain and thus caused a long cold winter. Nearly all life that was dependent on sunlight died and complete food chains collapsed.

The first decade after this publication, far from everyone was convinced, but over the years an increasing amount of evidence for this impact was found, such as spherules of melted rock,   quartz crystals which were ‘shocked’ by the impact, and in 1990, as the last master card: the 250 km wide impact crater on the Yucatán peninsula in Mexico (Hildebrand et al., 1991). All together  there was such a large amount of evidence for the impact theory that almost the entire scientific community was convinced.

To this day, the Cretaceous–Paleogene sections that Luis and Walter Alvarez first examined - the Gubbio section in Italy, the Stevns Klint section in Denmark and also the Woodside Creek section in New Zealand-  are considered as some of the most important geological sections in the world.

Indeed, Stevns Klint is and remains a notable section. The relatively soft white limestone of the upper Cretaceous (Maastrichtian), with bands of flint tubers (probably the traces of  ‘Thalassinoides’ burrows) of the Sigerslev and Højerup Members of the Tor formation, contrasts with the dark Fiskeler Member and the Cerithium limestone Member of the Rødvig formation, the earliest Paleocene deposits.

The Tor Formation mainly consists of the remains of bryozoa reefs. Although aragonite skeletons usually aren’t preserved, the filling of the heteromorph ammonites Baculites and Scaphites can be found. The Tor formation was deposited in a dynamic lime sea, which was abruptly disrupted by the extinction of all the lime producing plankton on the Cretaceous–Paleogene boundary. Under the poor lime-and-oxygen conditions that followed, the dark, clayey and pyrite-rich marl of the Fiskeler could be deposited. At the deeper depressions in the Tor Formation one can find, on top of the Fiskeler, the Cerithium limestone Member, a whitish, hardened limestone-bank with a maximum thickness of about 50 cm.

The return of lime meant that after a break of thousands of years, the conditions in the  Danish limesea finally started to recover. The biggest contrast in Stevens Klint however, is between these formations and the Korsnœb Member of the Stevns Klint formation, consisting of hard limestone formed by so-called ' Bryozoan mounds '. These ' mounds ' are rich with fossils, which is a sign that the weather conditions were optimal again for lime-producing organisms.

If you look closely you can see that the Stevns Klint Formation clearly cuts into the Rødvig formation (in some places it even cuts into the Tor Formation). This has to do with a sea level decline that took place just after the Cretaceous–Paleogene boundary.

Since Luis and Walter Alvarez examined Stevns Klint, geologists are very interested in this section and have often returned to do new research, both to find more evidence for the meteorite as to investigate the environmental changes on the border. In recent years, too, Stevens Klint is still under attention. For example, in 1998 during a tour of first year geology students, there was a find which stunned paleontologists. On the beach between the Boesdal quarry and the sea, a piece of brown flint was found with two aptychi in it, the closing valves of an ammonite. The nearest lime deposits are about 400 m away and contain mainly black and grey flint.

So it is very likely that this flint originates from the lower Paleocene. The aptychi are very well preserved and are still together, which indicates that they are probably not reworked (Surlyk and Nielsen, 1999). More ammonites have been found in the Cerithium limestone (belonging to the types of Hoploscaphites constrictus and Baculites sp.), but these were hitherto always considered as Cretaceous reworked. The find of this flint seemed to show that ammonites in Denmark occurred above the Cretaceous–Paleogene boundary!

Of course this find was received with great skepticism. Didn’t ammonites die out on the Cretaceous–Paleogene boundary? Couldn’t this flint just have been reworked? After all, it was a beach find? However, in 2005 the paleontologists Marcin Machalski and Claus Heinberg published an article in which they demonstrated with carbon isotopes and micro fossils that the ammonites in the Cerithium limestone came without doubt from the earliest Paleocene, and have therefore survived the Cretaceous–Paleogene boundary.

Since then, the goal of a research project has been to investigate the occurrence of ammonites in the earliest Paleocene. Marcin Machalski and Claus Heinberg are active researchers in this project, as well as John Jagt from the Natural History Museum in Maastricht. This team also does research on ammonites above the Cretaceous–Paleogene boundary on other locations, including in the IVf-7 unit of the Meerssen Member in the Curfs quarry and the "Geulhemmerberg" in South-Limburg!

All-in all, Stevns Klint is a section well worth visiting! And who knows, if you look well, perhaps you find an ammonite from the Paleocene!

 

Thanks to Johan Vellekoop for writing this article.

 


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